Robert Michels's thesis that political parties and other large membership organisations inevitably tend to be controlled by a small leadership, whatever their democratic ideals.
In more detail
Michels set out the thesis in Political Parties (1911), arguing that organisational needs such as rapid decisions and full-time officials concentrate control. Later studies, including Lipset, Trow and Coleman's work on a typographers' union with internal party competition, show that the tendency can be checked. Most scholars treat it as a tendency rather than an iron law.